1. Field of the Invention
The present invention relates to a conductive paste, and more particularly to a conductive paste used for forming conductive vias within through holes of a ceramic substrate. The present invention also relates to a method for making a ceramic substrate in which through holes are filled with conductors vias.
2. Description of the Related Art
A ceramic substrate for mounting various kinds of electronic components often has through holes therein and conductive vias are formed in the through holes in order to electrically connect conductive patterns formed on opposing surfaces of the ceramic substrate. Conductive vias are also employed in some electronic components such as certain capacitors, inductors or composite electronic chips thereof.
Such ceramic substrate with conductive vias and conductive patterns has been prepared by the following method. Specifically, a plurality of ceramic green sheets provided with through holes by drilling or punching are first prepared. The through holes are filled with a conductive paste and a conductive paste is printed on the ceramic green sheets so as to form conductive patterns thereon. Thereafter, the ceramic sheets are stacked with each other by pressing, thereby forming a laminate. After the laminate is cut into pieces having a predetermined size, each piece of the laminate is sintered. During the sintering, the conductive paste in the though holes and the conductive paste printed on the ceramic green sheets are also sintered. Thus, prepared is a ceramic substrate which has conductive patterns electrically connected through the conductive vias.
A conventional conductive paste for forming the aforementioned conductive vias is generally composed of copper powder and an organic vehicle. Copper powder is employed because copper is an inexpensive material and is superior in electromigration and resistivity. However, the ceramic substrate prepared using the conventional conductive paste has the following problems. As shown in FIG. 1A, if the conductive paste shrinks more than the ceramic green sheet during the sintering, a void 13 or a crack 14 is formed in a conductive via 12 filled in a through hole 11 of the ceramic substrate 10. On the other hand, if the ceramic green sheet shrinks more than the conductive paste during the sintering, the conductive via 12 swells from the through hole 11 of the ceramic green sheet 10 or a crack 15 occurs along the periphery of the through hole 11, as shown in FIG. 1B. These defects cause disconnections between the conductive patterns in the ceramic substrate, thereby degrading the reliability of the ceramic substrate.
The inventors of the present invention have confirmed that a void 13 or a crack 14 is likely to occur due to insufficient filling of the conductive paste into the through hole or overshrinking of the conductive paste in the case where copper powder used in the conductive paste has an average diameter less than 2 .mu.m. It was also confirmed by the inventors that the conductive via swells from the through hole in the case where copper powder used in the conductive paste has an average diameter more than 2 .mu.m.
For the foregoing reasons, there is a need for a conductive paste which does not cause the aforementioned problems.